New form of superconduction observed in interfaces

Superconductivity was observed at the interface of two thin films that, in and …

Ars has brought you lots of coverage of research in superconductivity, including the discovery of a new class of superconductors and a new
theory that attempts to describe the phenomenon. As it's a branch of physics that is
poorly understood, research efforts often lead to new
observations of the phenomenon and new questions, rather than solutions to existing questions. A paper to be published in Nature today falls squarely into the new-observations category. Researchers have observed
superconductivity at the interface of two materials that are not inherently
superconducting at any temperature, suggesting that we can engineer
superconductors at small dimensions.

The generally accepted explanation for superconduction involves Cooper Pairs, where two electrons become weakly coupled at a relatively large distance (several nanometers) through an interaction with phonons (heat's quantum equivalent to the photon) that are vibrating in the crystal lattice. If one electron is impeded by a normal scattering site like an impurity or crystal imperfection, the other electron in the pair can "pull" it along.

The interaction is so weak that temperatures above 30K will break the pair, so this model works to describe pure metals that superconduct at temperatures below 10K. The theory breaks down for high-temperature oxide superconductors, which have achieved superconduction at temperatures as high as 138K (at atmospheric pressure). Clearly there are other mechanisms at work, but we don’t currently understand them.

The new research involved a lanthanum copper oxide compound that can be doped over a wide range of compositions, which was used to study a potentially new mechanism of superconduction. A substrate of LaSrCuO4 was used, and an epitaxy technique grew atomically-perfect thin films of three derivative compounds: an insulator and a metal that show no superconductivity, and a superconducting variant with a transition temperature (Tc) of 40K. By growing literally hundreds of combination of interfaces and film thicknesses, the researchers were able to observe superconduction at different temperatures, including superconduction at the metal/insulator interface.

The authors took great lengths to characterize the materials and confirmed that the interface was both atomically perfect and pure, meaning that a third material was not formed from inter-layer mixing. This means that an interface interaction is responsible for the superconduction. By varying the thickness of the films used, the authors found that the superconductivity was occurring in just two unit cells around the interface. Interfaces produced with a superconducting variant boosted its Tc from 40K to 50K, while interfaces with the two nonsuperconducting layers had Tc's as high as 30K.

While there is no definitive explanation available for this interfacial superconduction, it opens the door for further research into engineering superconductors out of non-superconducting materials. The small length scales at which the superconduction occurs may make it appropriate for micro- and nanoscale devices.